would you attempt that in your 20g tank? if so, most likely not. i tried a couple pair in my 75g reef, and my 50g nem tank without success. my neighbor has a tank with 5 clowns consisting of a pair of clarki and 3 ocellaris. he had more that that before i took one off his hands. lol they actually seem to get along decently and have been for over a year. that is a rare thing though, and chances are he will have issues at some point.
the best success rate that i have read about with groups of clowns is a large tank, lots of cover, and getting a larger group of at least a dozen small fry from the same batch. i tried a group of 5 ocellaris that were from the same batch and 1" in my nem tank. they eventually had issues and i had to separate.
personally, i would not attempt it again...